We use attosecond pulse trains to study the electronic dynamics occurring during the process of photoionisation of molecules on a time scale of tens of attoseconds. Although for many applications the photoionisation process can be assumed to be instantaneous, there are tiny but measurable time delays between the arrival of the pulse and the photoemission process, or between the photoemission between different electronic levels. These delays can be used to gain insight into the electronic structure and dynamics of the molecules under investigation.
Our experimental approach is based on the RABBIT technique developed by Nobel laureate P. Agostini and collaborators in 2001 (P.M. Paul et al. Observation of a Train of Attosecond Pulses from High Harmonic Generation. Science 292, 1689-1692 (2001)). In this technique, a train of attosecond pulses is combined with an infrared laser field. The photoelectrons generated in the laser-assisted photoionisation process are measured for different relative delays between the attosecond pulses and the infrared field.
Our experimental setup for the measurement of photoelectron spectra obtained with the RABBIT technique is based on the combination of a high repetition rate (50 kHz) attosecond source with a photoelectron-photoion coincidence spectrometer. This approach, although extremely demanding from an experimental point of view, provides a great physical insight into the photoionisation process by allowing the full characterisation of the emission direction of the photoelectron wave packet (energy and angle resolved information). Moreover, in combination with ion detection, photoelectrons associated with different cationic and even dissociative states can be studied independently.
Using this approach, we have demonstrated the reconstruction of the effect of shape resonance in the photoionisation of CF4 molecules. The measurement of the CF3+ recoil fragment allows us to reconstruct the molecular orientation at the moment of ionisation and to assign all photoelectrons to this frame.
Coincidence spectroscopy also offers the opportunity to study attosecond time delays in molecules with different isotopic substitutions. Using a mixture of methane and deuteroethane, we have shown that nuclear dynamics can profoundly affect the photoelectron spectra emitted by the two molecules, with important implications for the coherence properties of the emitted photoelectrons.
More generally, we aim to investigate how the complex molecular energy landscape affects the properties of the photoelectrons leaving the molecular system.
[1] H. Ahmadi et al. Attosecond photoionisation time delays reveal the anisotropy of the molecular potential in the recoil frame. Nature Communications 13, 1242 (2022).
[2] D. Ertel et al. Influence of nuclear dynamics on molecular attosecond photoelectron interferometry Science Advances 9, eadh7747 (2023)
[3] D. Ertel et al. Anisotropy parameters for two-color photoionization phases in randomly oriented molecules: theory and experiment in methane and deuteromethane The Journal of Physical Chemistry A 128 1685-1697 (2024)